A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processor 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processor 111 into a digital data format, such as a sequence of data frames, for transmission into a receiver processor in a stimulator processor 108. Besides extracting the audio information, the receiver processor in the stimulator processor 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through connected wires 109 to an implant electrode 110. Typically, the implant electrode 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
The stimulator processor 108 typically does not have an internal battery so the radio-frequency inductive link signal from the external signal processor 111 is used both for transferring data (for example, for stimulation) and also to provide energy for powering the implanted components. This raises a number of technical challenges. The transmission of energy to the implanted components should be power efficient and independent of data transmission without impairing the overall system performance. In addition, the implant supply voltage is affected by many factors such as electrode load impedance, inductive link coil coupling distance, number of stimulated channels, stimulation amplitude, and stimulation duration. Notwithstanding these many factors, the implant supply voltage should be as stable as possible to generate reproducible stimulation pulses. The implant supply voltage also needs to exceed a predefined minimal supply voltage VTHR for some minimal defined initialization period. The stimulator processor 108 also must scan the inductive link signal for data words and decode those correctly. These considerations vie with the need for low power consumption since the implant supply voltage is not needed when no stimulation is produced and energy losses increase (at least linearly) with the supply voltage.
In general, a stable voltage supply is achieved by voltage regulation, which can be realized by feedback from within the implant (regulator with internal feedback) or by telemetry of the supply voltage status to the external components which regulate the internal supply voltage from outside (regulator with external feedback). See, e.g. Mark van Paemel, High-Efficiency Transmission for Medical Implants, IEEE Solid-State Circuits Magazine, Digital Object Identifier 10.1109/MSSC.2010.939572; incorporated herein by reference. One disadvantage of these regulation schemes is the additional circuitry that decreases the overall circuit reliability and which itself consumes energy. For example, the simplest but very energy inefficient regulator is a Zener-diode.
As mentioned the stimulator processor 108 typically is not equipped with batteries but instead uses load capacitors for energy storage. This means that the implant cannot store large amounts of energy and so any excess transferred energy will be wasted. Typically the inductive link signal uses a Manchester coded ASK (Amplitude Shift Keying) signal, which ensures that both data and energy are continuously transferred to the stimulator processor 108 whenever data needs to be delivered. When no acoustic information needs to be transferred, redundant Manchester coded data can be sent. For example, logical 1's or 0's are represented in a sequence of [RFoff/RFon] or [RFon/RFoff] respectively. Sending redundant Manchester coded data is relatively easy to implement, but that brings the disadvantage that the same amount of energy is transferred to the stimulator processor 108 and surrounding tissue even though less energy is needed since no data is transferred and hence less power is consumed.